Direct-positive silver halide emulsions containing silver halide reducing agents in a low ph environment

ABSTRACT

DIRECT-POSITIVE COLLOID-SILVER HALIDE PHOTOGRAPHIC EMULSION LAYERS CONTAINING A SILVER HALIDE REDUCING AGENT IN A LOW PH ENVIRONMENT AT ALL TIMES PRIOR TO IMAGEWISE EXPOSURE.

United States Patent Otfice 3,560,213 Patented Feb. 2, 1971 DIRECT- POSITIVE SILVER HALIDE EMULSIONS CONTAINING SILVER HALIDE REDUCING AGENTS IN A LOW pH ENVIRONMENT Heman Dowd Hunt, Eatontown, N.J., assignor to E. I. du Pont de Nemours and Company, Wilmington, Del. No Drawing. Filed Sept. 12, 1967, Ser. No. 667,077

Int. Cl. G03c 5/24 U.S. Cl. 96-64 Claims ABSTRACT OF THE DISCLOSURE Direct-positive colloid-silver halide photographic emulsion layers containing a silver halide reducing agent in a low pH environment at all times prior to imagewise exposure.

BACKGROUND OF THE INVENTION Field of the invention The invention pertains to photographic silver halide layers capable of forming a direct-positive image embodying a silver halide reducing agent in an inert environment but which can be activated at a desired later time.

Description of the prior art In Hunt US. Pat. 3,033,678, May 8, 1962, light developable, direct-print emulsions which contain stannous chloride are disclosed. These emulsions are not at a low pH during addition of the stannous salt, and fogging occurs as the stannous salt is not present at all times in an inert environment prior to exposure and processing. In related Hunt US. Pat. 3,189,456, June 15, 1965, a sensitizing agent is added to the emulsion but the stannous salt is not at all times in an inert environment, and directpositive emulsions are not formed.

At p. 161 of The Theory of the Photographic Process, 3rd ed., C. E. K. Mees and T. H. James, The Macmillan Co., New York, an effect under the title Internal-Image Densitization is discussed which, it is believed, may aid in understanding the present invention. Related effects are obtained by replacing a second light-exposure by a chemical fogging treatment, e.g., the aerial fogging disclosed in Fallesen US. Pat. 2,497,875, Feb. 21, 1950, and by adding materials, e.g., benzotriazoles, to restrain growth of the negative image, as disclosed in Stauffer US. 2,497,- 917, Feb. 21, 1950. None of these references, however, teach the incorporation into the emulsion of a silver halide reducing agent capable of causing formation of chemical fog centers, but under conditions such that the reducing agent is always in the presence of silver halide grains in an inert (non-fogging) environment.

SUMMARY OF THE INVENTION This invention relates to a light-sensitive, photographic, colloid-silver halide emulsion layer containing a silver halide reducing agent in a low pH environment in which said agent, while in the presence of silver halide, is inert at all times, i.e., the agent is unable in such an environment to cause the formation of chemical fog centers or sensitivity centers on the silver halide grain surfaces.

The invention also relates to a process for forming a direct-positive image.

(1) By imagewise exposing the above-defined silver halide emulsion layer, and

(2) By treating the exposed layer to change the environment of the silver halide reducing agent so that the agent in the presence of silver halide grains no longer is in an inert environment but is able to cause formation of chemical fog centers on the surfaces of unexposed silver halide grains. Step (2) can be accomplished by developing the unexposed grains by treatment with an alkaline photographic silver halide developing solution which raises the pH so that the reducing agent becomes active and has a fogging action on the unexposed silver halide grains. The particular pH below which the reducing agent is inactive varies, but in the case of stannous chloride, the preferred agent, is usually below 3.5 and usually above about 1.0.

The direct-positives are obtained in accordance with this invention by the reducing agent becoming active so that unexposed silver halide grains become chemically fogged so as to become developable without the addition of other fogging agents (such as air to cause aerial fogging) as required in the prior art references.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In carrying out the invention, a reducing agent such as a stannous salt is incorporated with a colloid-silver halide emulsion while maintaining an environment in which the reducing agent is inert from the time of its addition until after completion of the imagewise exposure. For example, the emulsion may be maintained at a relatively low pH until after the exposure. In the case of the preferred agent, stannous chloride, which is an acid salt and has a marked effect on the pH, the pH should be maintained at a value no higher than about 3.0 to 3.5.

For best results, the photosensitive layers should be of an internal image emulsion which comprises silver halide grains having a substantial amount (preferably a predominant amount) of sensitivity internal to the grain. Such emulsions are discussed in chapter 6 of the above cited Mees textbook which refers to developer formulae for surface-type and internal-type photographic developer solutions along with discussions of typical behavior of internal image emulsions in various developer formulations. Those skilled in the art of manufacturing photographic emulsions are familiar with the various means of increasing the ratio of internal to surface sensitivity, including the minimizing of chemical sensitization of the silver halide surface.

In a preferred embodiment of the invention, the emulsion should include a protective agent which adsorbs to the silver halide surface and diminishes the opportunity for the reducing agent, in its required environment, to react with the silver halide. Pronounced effects can be obtained with the addition of protective agents such as metal iodides, molecular iodine, photographic spectral sensitizing dyes, e.g., cyanines, carbocyanines, and merocyanines, or certain mercaptans (especially 2-mercapto-4- phenylthiazole). These protective agents are helpful in preventing development of silver halide in exposed areas so as to give better minimum densities and, therefore, improved visual contrast. Molecular iodine is preferably used in an amount of 0.002 to 10 g. per mole of silver halide while a spectral sensitizing dye is used in an amount from 0.01 to 1.0 g. per mole of silver halide.

While gelatin the preferred binding agent for the silver halide grains, other natural and synthetic water-permeable macromolecular organic colloid binding agents can be substituted for the gelatin. The specific colloid binding agents described in Hunt 3,033,682 can be used in accordance with this invention.

Suitable silver halide reducing agents are metal compounds, including dicyclopentadienyl iron, titanium chloride, stannous chloride, and other agents which during development cause fogging of silver halide grains having no latent images but do not fog grains containing an internal latent image.

Metal ions, e.g., aurous or auric, can be present in the emulsion so that the reducing agent, activated by the 3 changed environment at processing, has a highly reducible ion that acts as an eflicient fog center. Gold ions also increase speed and lower the minimum density by sensitizing the internal latent image response.

The emulsions may contain any of the usual adjuvants such as coating aids, gelatin hardening agents viscosity modifiers, matting agents, pigments, non-halation dyes, antifoggants, covering power agents, dispersed polymeric latices, etc. Chemical sensitizers may be present although in preferred emulsion these are omitted so the surface sensitivity will be minimized relative to that of the internal sensitivity.

The emulsion can be coated on any suitable support, e.g., those disclosed in Nottorf US. 3,142,568. The elements prepared from these emulsions may also comprise various auxiliary layers as disclosed in the Nottorf patent.

In the preferred embodiment wherein a stannous salt is used as the reducing agent, its concentration should be from .001 to 100 mole percent based on the silver halide.

The invention will be further illustrated but is not intended to be limited by the following examples wherein, unless otherwise stated, silver halide coating weights are calculated in terms of the equivalent weight of pure silver bromide.

EXAMPLE I A gelatino-silver chlorobromide emulsion was prepared by precipitating silver chloride crystals in an aqueous gelatin solution and then subjecting these crystals to Ostwald ripening in the presence of 1.6 moles of potassium bromide per mole of silver chloride. The silver halide crystals thus prepared (according to procedures well known in the art) were of the type having a strong tendency to form internal latent images. After coagulation and washing to remove soluble salts, as taught in Moede US. 2,772,165, the emulsion was found to contain silver halide which was about 94 mole percent silver bromide and about 6 mole percent silver chloride. One mole of the washed emulsion was redispersed in an aqueous gelatin solution of such concentration that the resultant emulsion contained approximately 40 g. of gelatin per mole of silver halide. Subsequently, a mixture of dye solutions was added consisting of 490 ml. and 533 ml., respectively, of ethanol solutions of dyes A and B, each solution containing 1 g. of dye in 2000 ml. The dyes were:

(A) 3,3-diethyl-5,5-dimethyl-9-methy1thiacarbocyanine-p-toluene sulfonate salt, and (B) 3,3'-diethyl thiacyanine iodide.

The emulsion was then digested for 40 minutes at 140 F. and the following solutions were added:

Chrome alum hardener was added, the emulsion was diluted with water to the desired level, and the pH was recorded as about 2.0. The emulsion was applied to a photographic paper base at a silver halide coating weight of 30 mg./dm.

A strip of the dried coating was exposed to a 28-m.c.s. (meter candle second) xenon flash for 10- seconds through a photographic step wedge which differed in optical density by a factor of two between adjacent steps. Next, the strip was developed for 2 minutes at 70 F. in a tray that was gently shaken and contained a solution of the following composition:

Processing was continued at 70 F. with a 1-minute treatment in an aqueous 5% by weight acetic acid solution, then a 5-minute treatment in a commercial, acid, white alum fixing solution (Du Pont F Fixing Bath, Photo Lab Index, p. 5-26, 1959), and finally a 5-minute wash in tap water. The resultant direct-positive image had a maximum reflection optical density (D of 1.22, minimum density (D 0.01, a long straight line portion of the sensitometric curve and a speed such that the density was reduced to 0.2 at an exposure equivalent to about 0.2 m.c.s.

EXAMPLE II An emulsion containing one mole of silver halide was prepared in the manner described in Example I except that 200 g. of gelatin were added after washing and the emulsion adjusted with HCl at 1 millimole HCl per gram of added gelatin. The emulsion was vigorously digested at 160 F. for min. and final additions were made which included 0.2 mole SnCl -H O, 0.02 mole Pb(NO and chrome alum hardener. This emulsion (pH 3.0) and free from dye, was cooled to 110 F. and coated on photographic paper at a silver halide coating weight of 32 mg./dm.

Exposure and processing were in the manner of Example I with the following sensitometric results being obtained:

Speed was estimated to be the same as in Example I.

EXAMPLE III An emulsion was prepared as described in Example I except that 200 g. of gelatin per mole of silver chlorobromide were added after washing. The emulsion was a pH of about 6.0. After addition of 0.15 g. of 4-phenyl-2- mercaptothiazole per mole silver halide, the emulsion was digested min. at 160 F. At F. other additions were made, which did not include addition of a sensitizing coated on photographic paper at a silver halide coating weight of 30 mg./dm. The coating pH was 1.45.

Exposure and processing were in the manner of Example I except that development time was 1 min. Sensitometric results were:

White light speed was estimated to be the same as in Example I.

EXAMPLE IV An emulsion was prepared in the manner described in Example I except that 340 g. gelatin were added per mole of silver halide after washing. A merocyanine dye of the structure H2 C S I 1'12 0 N 0 H3 /O: s

ii 0 02m was added at a concentration of 500mg. per mole of silver halide and the emulsion was digested 20 min. at

After cooling to 110 F. the following were added:

Mole 'Pb(NO 0.02 HCl 0.32

SnCI -ZH O 0.40

Chrome alum hardener was added and the emulsion was coated on a paper at a silver halide coating weight of about 30 mg./dm. The emulsion had a pH of about 2.5.

Exposing and processing in the manner of Example I resulted in with a white light speed about the same as in Example 1.

EXAMPLE V An emulsion was prepared similar to that of Example I except that 560 g. of gelatin per mole of silver halide were present. Instead of stannous ion, ferrocene (dicyclopentadienyl iron) was used at a concentration of 6.7 g./ mole of silver halide. This compound acts as the fogging agent which is contained in an inert environment until completion of exposure. Other additions were:

AcidNone Dye of Example IV560 mg. per mole of silver halide Pb(NO 0.02 mole per mole of silver halide.

Chrome alum hardener was added and the emulsion was coated at a pH of 5.45 on paper at a silver halide coating weight of 30 mg./dm.

Processing gave a direct positive image of lower density than obtained in those examples where a stannous salt was used as the fogging agent but it was determined that the speed was equivalent. In this example the reducing agent, ferrocene, was in an inert environment with the emulsion at a pH of 5.45. The developer solution, with a pH of 10.3, released the ferrocene from its inert environment so that it was able to form chemical fog centers in unexposed areas.

EXAMPLE VI An emulsion similar to Example V was prepared except that TiCl (6.7 g. per mole of silver halide) replaced the ferrocene.

Other additions were:

ACld NOHE Dye of Example IV560 mg./mole of silver halide Pb(N -0.02 mole per mole of silver halide.

Chrome alum hardener was added and a coating at mg./dm. of silver halide was made on paper base. The pH at coating was 4.55.

Processing gave a direct positive image with a high minimum density. The speed was about 10 times slower than other examples but this was largely due to the masking of gradations by the high minimum density.

EXAMPLE V11 A gelatin-silver bromide esmulsion was prepared by' precipitating silver bromide crystals from a solution containing 60 mole percent excess KBr over that necessary to precipitate the silver from aqueous AgNO in aqueous gelatin, there being 33 g. of gelatin per mole of silver halide. The system was acidic due to the addition of 0.054 mole of HCl per mole of silver halide. The silver halide crystals were ripened and coagulation washed. The curds were redispersed and gelatin added to give a total gelatin concentration of 560 g./mole of silver halide. Dyes were added to give, per mole of silver halide:

Mg. Dye A (of Example I) 247 Dye B (of Example I) 267 After 40 min. digestion at 140 F. the emulsion was cooled to 110 'F. and there were added, per mole of silver halide,

Mole

HCl 0.32 Pb(NO 0.02

SIlClg' Chrome alum hardener was added and emulsion was coated on paper at 30 mg./dm. of silver halide.

Exposure and processing in the manner of Example I gave the following results:

White light speed was equivalent to that of Example I.

EXAMPLE VIII 10 A gelatino-silver iodobromide emulsion containing 7 mole percent silver iodide and 93 mole percent silver bromide was prepared in a manner well known to the art. This emulsion does not normally form internal latent images when fully sensitized as a high speed negative Chrome alum was added and the emulsion coated on paper base at 30 mg./dm. of silver halide. Exposure and processing in the manner of Example I gave a weak image but one in which the white light speed was about that 30 of Example I.

EXAMPLE IX Emulsions A and B were prepared essentially in the manner of Example I except that the plumbous ion was omitted in B. Stannous chloride was present at 0.24 mole per mole of silver halide in both emulsions.

Coating, exposure and processing were as described in Example I, and gave the following results:

D mu; 1. 22 0. 70 Dmin- 0.14 0.14 Positive shoulder speed,* in c s 0 014 0. 010 Positive toe speed, m.c.s 0. 44

Negative toe speed,*** m.e.s.

Lowest exposure level at which optical density began to be reduced from Dm Value.

*" Lowest exposure level at which optical density reached Dmin value.

*** Lowest exposure level at which negative image began to appear.

The effect of plumbous ion was to suppress negative (surface) sensitivity and promote fogging by stannous ion to give a more pronounced D The plumbous ion also has the advantage of effectively reducing the negative speed to give a wider exposure latitude for forming the desired positive image.

EXAMPLE X An emulsion similar to that of Example I contained one mole of silver halide. Gelatin was added to give a total of 375 g. and 475 mg. of dye B of Example I was added. The emulsion was digested 20 min. at 140 F. and the following additions were made after cooling to 110 F.:

Mole

HCl 0.32

PbNO 0.02 SnC1 0.05 SnCl -2H O 0.36

This emulsion was coated at 35 mg/dm. of silver halide on a resin subbed polyester film base as described in Example IV of U8. Pat. 2,779,684 which had been overcoated with a thin gelatin anchoring layer. Exposure and processing were as in Example I except that the temperature was F. and the film was developed seconds. A direct positive was obtained although the D was substantially lower than in coatings on paper base.

Stannic ion promoted D The principal of mass action and the low pH of 2.20 achieved by use of stannic chloride stabilized the stannous ion in regard to oxidation. The following sensitometric results were obtained:

D (transmission) 0.30 D (transmission) 0.07 Positive toe speed, m.c.s. 0.9 Negative toe speed, m.c.s. 5.0

EXAMPLE XI An emulsion containing one mole of silver halide was prepared in the manner of Example I except that after washing the curds were redispersed in a solution containing 100 g. gelatin. Other additions were:

Mole

Plumbous nitrate 0.020 Hydrochloric acid 0.064 Molecular iodine 0.004 Stannic chloride 0.002 Stannous chloride 0.045

G. Na SO anhydrous 65 Hydroquinone 10 Na CO -H O 1-phenyl-3-pyrazolidone l Benzotriazole 1 Water to make 1 liter.

The paper was then rinsed and fixed.

A high speed direct positive resulted (in the absence of spectral sensitizing dyes) with the following sensitometric properties D 0.79 min 0.21

Positive shoulder speed, m.c.s 0.01

Positive toe speed, m.c.s. 1.8

Negative toe speed, m.c.s 3.5

EXAMPLE XII Three emulsions were prepared in the manner of Example XI except that, per mole of silver,

Emulsion A had 0.04 mole KI and no iodine. Emulsion B had 0.02 mole I and no iodide. Emulsion C had no iodine or iodide.

The pHs at coating were, respectively, 3.7, 3.3 and 3.5. Exposure and processing were as described in Example XI. Iodine gave the superior direct positive but all showed the effect.

Coating i emulsion A B 0 DMX 0. 75 0.77 0.77 D iu 0. 61 0. 26 0.72 Positive shoulder speed, m.c.s 0. 02 0. 01 0. 08 Positive toe speed, m.c.s 0. 9 1.8 3.

EXAMPLE XIII Two emulsions, each containing one mole of silver halide, were prepared as in Example I except that after redispersion in g. gelatin the following additions were Emulsion temperatures after redispersing in gelatin were held at 95 F. Hardener and coating adjuvants were added and the emulsions coated on paper base at 40 mg./ dm? of silver halide. Exposure and processing were carried out as in Example XI to give the following results:

Coating of emulsion A B Dmu l. 07 0. 0-1 Dam. 0. 99 0. 50 Positive shoulder speed, m.c.s 0. 4 0. 014 Positive toe speed, m.c.s 3. 5 3. 5

The auric ion improved the positive effect by reducing th D EXAMPLE XIV A gelatino-silver chlorobrornide emulsion was made by slowly adding an aqueous solution containing a mixture of 1 mole of silver nitrate and 0.033 mole of plumbous nitrate to a gelatin solution containing 1 mole of potassium chloride and acidified with 0.05 mole of hydrochloric acid. The precipitation was carried out under red safelights. The temperature at precipitation and for 40 minutes thereafter was held at 160 F. After precipitation an aqueous solution of 1.6 moles of potassium bromide was added while the mixture was held at 160 F. The resulting emulsion was coagulated, washed, and redispersed in accordance with the technique described in Moede, US Pat. 2,772,165. The curds were redispersed at a pH of 6.3 and 167 g. of gelatin were added slowly and stirred 20 minutes at F. as the gelatin melted. Molecular iodine was added (0.001 mole) along with sensitizing dye B of Example I (267 mg.) and 67 mg. of a carbocyanine dye The emulsion was then digested at F. for 20 minutes. A wetting agent (saponin) and the other additions following were then made at 95 F.:

Chrome alum7 g.

Stannous chloride0.12 mole Aqueous borax solution-to give pH 3.2 Plumbous nitrate0.02 mole Potassium bromide0.30 mole.

The emulsion was coated on paper at 30 mg./dm. of silver halide. Exposure and processing as in Example XI yielded a high speed direct positive image with the following sensitometric data:

D 0.44 D 0.20 Positive shoulder speed, m.c.s 0.01 Positive toe speed, m.c.s 0.22

Negative toe speed, m.c.s. 1.0

EXAMPLE XV Silver halide crystals were prepared in the manner of Example 1. After washing, the silver halide was redispersed in an aqueous solution of gelatin containing 133 g. gelatin per mole of silver halide. Portions of the emulsion were treated with hydrochloric acid as shown in the table. To each portion there was then added 467 mg. dye/mole Ag (the dye described in Example XIV) and 3,3-dietl1yl-5,5-diplicnyl-9etliyloxacarbocyanine bromide.

the emulsion portions were digested at 140 F. for 20 minutes. At 95 F. there were added 0.02 mole Pb(NO 0.20 mole of stannous chloride and chrome alum hardener. Coatings on paper were made at 50 mg. AgBr/dm. The table shows that acid lowered the pH considerably and that stannous salts lowered it further. The directpositive effect was improved by the addition of acid.

Sensitometric testing was in the manner of Example I except that development was for 1 minute at 70 F.

10 4. An emulsion layer according to claim 1 wherein said agent is stannous chloride and the environment is at pH 3.5-1.0.

5. An emulsion layer according to claim 1 wherein 5 said emulsion is a gelatino-silver chlorobromide emulsion of the Strong internal sensitivity type, the agent is stannous chloride and the environment is at pH 3.5-1.0.

6. An emulsion layer according to claim 1 wherein the stannous chloride is present in an amount from .001 to 100 mole percent based on the silver halide.

pH after pH after Energy to Mole SllCiz give D i 1101 addition addition D Dmln m.c.s.

In this series, an optimum pH occurred at 1.45. 7. An emulsion layer according to claim 1 wherein a The effect of acid was to prevent the premature fogging or molecular iodine or metal iodide protective agent is action of stannous ion before the processing stage. Acid present. conditions keep the stannous ion inert and it is reduced 8. An emulsion layer according to claim 1 wherein a state until introduced into the basic developing solution. photographic spectral sensitizing dye is present.

The present invention provides novel emulsion layers 9. A process for forming a direct-positive image which capable of forming direct-positive images by simple and 30 comprises conventional processing. By the use of these emulsions (1) imagewise exposing to actinic radiation a photoaccording to the photographic processing procedures graphic element bearing a silver halide emulsion taught herein, it is possible to obtain reasonably fine layer as defined in claim 1, and grain images of unusually high positive speed or sen- (2) treating the exposed silver halide layer to raise sitivity to exposing radiation. Definition of the directthe pH so that the reducing agent forms chemical positive images is at least as good as images obtained by the more complicated reversal processing and is superior to that in images obtained by processes involving silver transfer techniques.

The embodiments of this invention in which an exclusive property or privilege is claimed are defined as follows:

1. A photographic element comprising a support bearing a direct-positive photographic colloid-silver halide emulsion layer containing a silver halide reducing agent and having a pH of not more than 3.5 at all times while said agent is in the presence of the silver halide grains so that the agent at the low pH is unable to cause the formation of chemical fog centers on the surfaces of the silver halide grains prior to imagewise exposure.

2. An emulsion layer according to claim 1 wherein said colloid is gelatin.

3. An emulsion layer according to claim 1 wherein said agent is an inorganic metal compound.

fog centers on the unexposed silver halide grains. 10. A process according to claim 9, wherein the exposed silver halide layer is treated in an aqueous alkaline solution containing a photographic silver halide developing agent.

References Cited UNITED STATES PATENTS 2,401,051 5/1946 Crouse 96--107 r 3,367,778 2/1968 Berriman 96-107 3,418,122 12/1968 Colt 96-107 3,418,124 12/1968 Hunt 96-64 3,418,131 12/1968 Bigelow 96-107 0 NORMAN G. TORCHIN, Primary Examiner J. R. HIGHTOWER, Assistant Examiner US. Cl. X.R. 9694 

